Method for the treatment of bismuth-containing sulphide ores or concentrates of such ore

ABSTRACT

Disclosed is a method of pretreating sulphide ores or sulphide ore concentrates having high bismuth contents that have a disturbing influence on further processing of the ores or concentrates, such as to enable the ores or concentrates to be further processed for the recovery of their valuable metal contents or at least to facilitate such processing. The method is characterized by leaching the ore or the concentrate with sulfuric acid over a predetermined time period while supplying heat at a pH below 2, and thereafter separating from the leachate a leaching residue in the form of a product which is leaner in bismuth and more enriched with regard to its valuable metal content than the input material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of pretreating sulphide ores orsulphide ore concentrates that contain bismuth in concentrationsobstructive to further processing of the ores or concentrates, so as toenable the ores and concentrates to be processed for the recovery oftheir valuable metal contents, or at least facilitate such processing.

2. Description of the Related Art

Sulphide ores and sulphide ore concentrates exist which because of theirhigh bismuth contents either cannot be used for the recovery of theirvaluable metal contents or must at least be processed with great care,for instance diluted with bismuth-lean materials. The problems are mostnoticeable with material that shall primarily be processed to producerefined copper, either pyrometallurgically or hydrometallurgically,since in both cases there is carried out an electrolytic treatmentprocess in which bismuth has a highly disturbing effect and contributestowards the formation of sludge/slime with the risk of serious impurityinclusions. Furthermore, refined copper is required to include a maximumof 1 ppm bismuth. Bismuth content limitations are also found in respectof lead and tin products, which is 50-100 ppm in the case of lead and100-500 ppm in the case of tin.

The possibilities of handling bismuth in a pyrometallurgical copperproducing process, for instance, are limited, because bismuth is arelatively noble element which is found in an elemental form in nature.Bismuth can be primarily separated as gaseous sulphide during the firststages of the copper process, i.e. during the smelting and convertingstages, provided that good contact is obtained between the copper matteand gas and that the smelt has a high temperature. Bismuth extraction isnot favoured by high-grade matte and the degree to which suppliedprocess air is enriched with oxygen. Despite the aforesaid possibilitiesof reducing the bismuth content, separate bismuth purifying stages areoften carried out in a later process stage, for instance invacuum-refining or soda treatment process stages, although this oftenresults in poor bismuth yields. In the majority of copper smelters, themaximum permitted bismuth content of the copper anodes used in theelectrolytic refining process is as low as 50 ppm, so as to enable theaforesaid problems concerning sludge formations and the serious impurityinclusions caused thereby, implying bismuth contamination, among otherthings, can be avoided to the best possible extent. This means that theintake of bismuth to the copper process must be limited, and therewithalso the intake of bismuth containing material. It is even necessary toreject certain materials because although the extraction of bismuthduring the pyrometallurgical treatment process is relatively constant,it is much too low. As will be readily understood, attention must beaimed primarily towards productivity and process economy, which impliesboth high-grade matte and oxygen enrichment, which as before mentionedare not the best conditions under which bismuth is eliminated during thecopper process.

Pretreatment of copper concentrates has been proposed with the intentionof reducing different troublesome impurity contents of the concentrateprior to its further processing. Partial roasting can be used to reducethe arsenic content, but the bismuth content is only influencedmarginally. The arsenic content, antimony content and mercury contentcan be reduced by sodium sulphide leaching, although the bismuth contentis not influenced to any great extent by this treatment. InCA-A-1057310, Outokumpu have proposed pretreatment in a rotary furnaceat 750° C. in a sulphur vapour atmosphere. This process eliminates 100%of the arsenic, 50-60% of the antimony, but only 20-30% of the bismuthcontent.

In EP-B-0138794, Boliden have proposed a method of eliminating primarilySb from copper smelt material. The method includes a separatechlorinating stage with an essentially stoichiometric quantity ofchlorinating agent in a rotary furnace at temperatures of 450-750° C.,wherein Bi is also removed to a substantial extent. Bio-oxidativepretreatment, bio-leaching, for selectively leaching bismuth from copperconcentrates has also been proposed. The selectivity and utility ofbio-leaching to this end, however, would seem to be completely dependenton the mineralogical composition of the concentrate, and hence its useis limited to the treatment of certain specific concentrates offavourable composition.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method by whichbismuth can be eliminated from sulphide ores or sulphide concentrates toan extent and with a selectivity with regard to valuable metal contentsuch as to enable the material to be used generally for the pretreatmentof such materials such that said materials can be readily processed andtheir valuable metal content recovered.

This object is achieved with a method having the characteristic featuresset forth in the present invention. Thus, the ore or the concentrateconcerned is leached with sulfuric acid over a given time period at a pHbelow 2, while supplying heat. At the end of this predetermined timeperiod, leaching residue is extracted from the leachate in the form of aproduct which is more bismuth-pure than the input material and in whichthe valuable metal content has been enriched. The leaching process ispreferably carried out in a pH range of 0-1. In order to achieve thebest possible result, the leaching temperature will preferably exceed50° C. The leaching process is preferably carried out in a series ofmutually sequential leaching stages. The consumed leachate mayconveniently be treated with lime or limestone while adding air toprecipitate leached-out quantities of bismuth and possibly iron in anhydroxide form, together with the sulphate content as gypsum.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a flow sheet illustrating bismuth leaching in accordancewith one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known that concentrated sulfuric acid or diluted sulfuric acid andair are able to react with elemental bismuth to form Bi³⁺ ions, whereinsulfuric acid is reduced to SO₂. However, there are no earlierdisclosures in the literature to the effect that sulfuric acid wouldreact in the same way with sulfidic bismuth compounds. Neither is thisprobable due to the very strong affinity of bismuth to sulphur. Despitethis, leaching with sulfuric acid under certain specified conditionsnevertheless results in appreciable elimination of bismuth andfurthermore selective elimination with respect to valuable metalcontents must be considered remarkable and surprising. According to onetheory, this may be due to complicated substitution reactions in theminerals between bismuth and valuable metals such as copper and silver.In addition to occurring as elemental bismuth, bismuth is also present,e.g., in sulfidic copper ores in the form of such sulphide minerals as

    ______________________________________    atildite           Ag Bi S.sub.2    bismuth glance     Bi.sub.2 S.sub.3    benjamite          Pb (Ag, Cu) Bi.sub.2 S.sub.4    hammarite          Pb.sub.2 Cu.sub.2 Bi.sub.4 S.sub.9    galenobismutite    Pb Bi.sub.2 S.sub.4    emplecite          Cu Bi S.sub.2    aikinite           Pb Cu Bi S.sub.3    ______________________________________

The invention delivers bismuth yields of up to 90% and even higher,whereas leaching of a valuable metal such as copper is not greater thanat most 2%. Iron, on the other hand, is leached out to a greater extent,resulting in a yield of up to 5-6%.

Thus, the inventive pretreatment process results in a product which incomparison with the input concentrate is purer with regard to Bi (andeven to Sb and As to some extent) and is also in applicable casesenriched with respect to valuable metals, since any iron present iseliminated although albeit to a limited extent. The pretreated metalwill thus be particularly attractive from several aspects with regard toprocessing its valuable metal content.

The invention will now be described in more detail partly with referenceto a flow sheet illustrating a preferred embodiment of the inventionincluding solution purification, and also with reference to an exampleillustrating leaching tests carried on a laboratory scale.

The accompanying drawing is a flow sheet illustrating bismuth leachingin accordance with the invention. Copper concentrate is taken from athickener or filter with the relevant water content and, optionallyafter being diluted with additional water, is passed to a first leachingtank with a solids content of 50-70%. Sulfuric acid is added to thetank, so as to maintain the pH at about 1. The contents of the tank areheated with heat taken from an external source. A suitable temperatureis 90° C. The tank contents are continuously transferred to a secondtank connected in series, and from there to a third and a fourth tank.The volume of the tanks and the residence time in respective tanks arechosen with regard to the predetermined total leaching time. Thisleaching time is chosen, in turn, on the basis of the yields desiredfrom the concentrate composition concerned. If suitable and if desired,SO₂ can be added to the last tank to precipitate leached copper. After asolid/liquid separation+washing stage, there is extracted an enrichedand bismuth-free copper concentrate for further processing in a smelter.As shown in the flow sheet, the leachate and added washing water isdivided into two parts, wherein Bi³⁺, Fe²⁺ /Fe³⁺ present in one part isprecipitated with limestone while supplying air at a pH in the region of3-5. Precipitate with bismuth-hydroxide, iron-hydroxide and gypsum isseparated downstream of the solids/liquid separation+washing stage.Residual solution, which may still contain copper in the form of Cu²⁺ions, can be returned to the leaching stage as a liquid addition. Coppercan precipitate from a bleed taken from this return flow, if so desired.The remainder of the leachate is returned to the first leaching tank asprocess liquid.

EXAMPLE

Copper concentrate from Aitik, which consists chiefly of the mineralschalcopyrite and pyrite, was leached in a series of tests. The bismuthcontent was 120 g/t and existed in the form of several differentminerals, namely the earlier mentioned matildite, bismuth glance,benjaminite, hammarite, galenobismutite, emplecite and aikinite. Asmaller part of total Bi was present as elemental bismuth.

The tests were carried out at a pulp density of about 45% solids. Theleaching procedure was studied under different conditions for twentyhours. 500 g concentrate and 610 ml of crude water from Aitik were mixedin each test. The pH was checked continuously during the tests andadjusted with H₂ SO₄ or NaOH. Leachate samples were taken at five timepoints: 0.5, 1.0, 2.0, 5.0 and 24 hours.

Test conditions and results have been entered in the Table below:

                  TABLE    ______________________________________    Test        Temperature                           Bi yield % Cu yield %    No.  pH     °C. 2h   5h   24h  2h   5h   24h    ______________________________________    1    0      25         36   35   35   0.3  0.4  0.7    2    1.0    25         28   26   34   0.2  0.3  0.4    3    0      90         70   80   83   0.6  0.9  1.7    4    1.0    90         72   80   92   0.3  0.4  0.6    5    0.5    55         40   54   75   0.4  0.6  1.3    6    0.5    55         40   54   75   0.4  0.6  1.3    7    1.5    90         35   48   42   0.2  0.3  0.3    8    2.0    90          4    2    0   0.2  0.3  0.3    9    2.5    90          0    2    0   0.1  0.1  0    10   3.0    90          0    0    0   0    0    0    ______________________________________

Optimal bismuth leaching conditions thus existed at a pH of 0-1 and at atemperature of 90° C., where the yields were as high as about 70%already after two hours, whereas the copper yields were, at the sametime, very low, about 0.5%. The Bi-yields were never higher than about30% at room temperature. A temperature of 55° C. greatly increases theyields. Practically no leaching of either Bi or Cu is obtained at pH=2or higher.

Suitable leaching times for the type of concentrate concerned areestablished by appropriate similar laboratory tests, although restrictedto the most optimal conditions at chosen leaching temperatures. Thenecessary leaching time is governed by the highest residual bismuthcontent desired in the concentrate. It may even be relevant andeconomically beneficial to lower the bismuth content to a very low levelwhen possible with regard to possible valuable metal losses in thetreatment process. In other words, the leaching time can be determinedprimarily with regard to the market situation, i.e. the value ofconcentrate of different degrees of purity with regard to bismuth. Theinventive method can thus be adapted readily and neatly to currenttechnical or market requirements, even with respect to ores orconcentrates of the most varying compositions.

We claim:
 1. A method of treating a copper-containing metal sulphide oreor copper-containing metal sulphide ore concentrate that containsbismuth as an impurity to reduce the bismuth content thereof comprisingcontacting the ore or concentrate with sulfuric acid at an elevatedtemperature and a pH of below 2 to preferentially leach bismuth from theore or concentrate and so as to form a leachate containing the leachedbismuth and an upgraded ore or concentrate, and separating the leachatecontaining the leached bismuth from the upgraded ore or concentrate. 2.The method of claim 1 wherein the ore or concentrate is contacted withthe sulfuric acid at a pH of 0-1.
 3. The method of claim 2 wherein theore or concentrate is contacted with the sulfuric acid at a temperatureabove about 50° C.
 4. The method of claim 3 wherein the ore orconcentrate is contacted with the sulfuric acid at a temperature aboveabout 90° C.
 5. The method of claim 3 wherein the ore or concentrate iscontacted with the sulfuric acid in two or more sequential steps.
 6. Themethod of claim 1 wherein the ore or concentrate is contacted with thesulfuric acid at a temperature above about 50° C.
 7. The method of claim6 wherein the ore or concentrate is contacted with the sulfuric acid ata temperature above about 90° C.
 8. The method of claim 1 wherein theore or concentrate is contacted with the sulfuric acid in two or moresequential steps.